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<center><h3>Gases in Chemical Equilibria</h3></center>

<p>Any species with a name ending with &#147;(g)&#148; is considered to be a gas.</p>

<p><nobr><b>log<i>K</i>&deg;-value:</b></nobr> for an equilibrium between a gas phase and an aqueous
solution, it must be assumed either</p>
<ul>
<li>constant pressure, or</li>
<li>constant volume.</li>
</ul>

<p>The value of <nobr>log<i>K</i>&deg;</nobr> will be different for these
two cases. For example,</p>
<center><nobr>CO<sub>2</sub>(g)</nobr> <nobr>+&nbsp;H<sub>2</sub>O</nobr>
  <img src="images/rlh.gif" alt="=" border="0" width="14" height="10">
  <nobr>HCO<sub>3</sub><sup>&#8722;</sup></nobr> <nobr>+&nbsp;H<sup>+</sup></nobr></center>

<p>For constant pressure:</p>
<center><nobr>log<i>K</i><sub>p</sub></nobr> = <nobr>log[HCO<sub>3</sub><sup>&#8722;</sup>]</nobr>
				<nobr>&#8722;&nbsp;pH</nobr> <nobr>&#8722;&nbsp;log<i>P</i><sub>CO<sub>2</sub></sub></nobr></center>
<p style="margin-top:0;">where <i>P</i> is in units of bar.
In the Earth&rsquo;s atmosphere <nobr>log<i>P</i><sub>CO<sub>2</sub></sub></nobr>
  <nobr>&#8776;&nbsp;&#8722;3.5</nobr></p>

<p>For constant volume:</p>
<center><nobr>log<i>K</i><sub>v</sub></nobr> = <nobr>log[HCO<sub>3</sub><sup>&#8722;</sup>]</nobr>
		<nobr>&#8722;&nbsp;pH</nobr> <nobr>&#8722;&nbsp;log[CO<sub>2</sub>]</nobr></center>
<p style="margin-top:0;">where the concentrations are given in mol/L.
Using the eqn. for perfect gases <nobr>(<i>P&nbsp;V</i></nobr> =
<nobr><i>n&nbsp;<a href="1_Some_Constants.htm">R</a>&nbsp;T</i>)</nobr> it is found that:</p>

<center><nobr><i>P</i><sub>CO<sub>2</sub></sub></nobr> =
    <nobr>[CO<sub>2</sub>]&nbsp;0.01&nbsp;<i>R&nbsp;T</i></nobr></center>

<p>The factor 0.01 is needed to convert the units <nobr>m<sup>3</sup></nobr> to L and Pa to bar.
Finally one obtains:</p>

<p><center><nobr>log<i>K</i><sub>p</sub></nobr> = <nobr>log<i>K</i><sub>v</sub></nobr>
			<nobr>+&nbsp;log(0.01&nbsp;<i>R&nbsp;T</i>)</nobr></center></p>

<p><b>Important:</b> DATABASE and SPANA are based on
<nobr>log<i>K</i><sub>p</sub></nobr> values for gases, with a standard state of <nobr>1&nbsp;bar</nobr>
<nobr>(&#8776;1&nbsp;atm).</nobr></p>

<p><a name="fugacity"></a>
Note also that only the aqueous and solid phases are considered when making
a diagram, and the concentration (in mol/L) for gases is set to zero.
For example, <nobr>CO<sub>2</sub></nobr> does <u>not</u> appear in
<a href="SP_Diagr_Fract.htm">fraction</a> or
<a href="S_Diagr_Log_CO2.htm">logarithmic</a> diagrams.
The <i><u>fugacity</u></i> <nobr>(&#8776;</nobr> partial pressure) of gases is calculated and
it is displayed in <nobr><a href="S_Diagr_Loga_CO2.htm">log activity</a></nobr> diagrams.</p>
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